1. Field of the Invention
The present invention relates to an inkjet printing apparatus and an inkjet printing method, and more particularly, to a configuration to reduce density unevenness caused by beading by way of arranging ink dots that constitute an image.
2. Description of the Related Art
In the field of inkjet printing, a so-called multi-pass printing method has been broadly known in which printing of an area is completed by scanning the same area with a printing head a plurality of times between which conveyance of a predetermined amount of a printing medium is performed. According to this printing method, a high quality image can be printed by reducing density unevenness caused by variation in the ink ejection characteristics among a plurality of nozzles that are provided on the printing head. In the multi-pass printing method, generally, the greater the number of scans (the number of passes) there are to complete the printing, the greater the variation in the ink ejection characteristics among the nozzles, as described above, is dispersed and therefore the higher the image of quality is printed. However, the greater the number of the passes is, the more time is needed to complete the printing of an area.
On the other hand, a problem of density unevenness or color unevenness has been known in an ink jet printing as caused by beading such that ink droplets are united on a printing medium such as printing paper before permeating thereinto. Various measures have been proposed toward this problem. In Japanese Patent Laid-Open No. 2007-306549 and Japanese Patent Laid-Open No. 2008-207385, a method of reducing density unevenness or color unevenness caused by the beading is described in which ink dots are arranged to be avoided uniting to each other on the printing medium.
However, the effect of reducing unevenness may not be exhibited sufficiently in the multi-pass printing even when the method of reducing density unevenness that is described in Japanese Patent Laid-Open No. 2007-306549 and Japanese Patent Laid-Open No. 2008-207385 is employed.
More specifically, a higher speed has been desired in an inkjet printing apparatus in recent years, and because of this, one of the measures considered has been to decrease the number of passes in the multi-pass printing. In this case, when the multi-pass printing is performed with a small number of passes such as two passes, the arrangement of the dots that are formed by the ejection of the ink for expressing a high density portion where an image density is relatively high comes to have a high density. Specifically, the ink droplets are ejected on each of pixels that are adjacent to each other in almost all of the pixels on which the dot is to be formed by the ejection of the ink. As a result, beading such that ink droplets are united may occur on almost the entire area of the printing medium. In such condition of which the ink droplets are applied with high density, beading may not be suppressed or reduced sufficiently even when the technique disclosed in Japanese Patent Laid-Open No. 2007-306549 and Japanese Patent Laid-Open No. 2008-207385 is employed.
On the other hand, when the ink is applied onto the printing medium, the ink permeation speed on the portion where the ink is applied decreases in general. Therefore, when the multi-pass printing with a small number of passes is performed, a distribution of a permeation speed may occur in which the permeation speed becomes low in the portion where the beading occurred in the prior pass and the permeation speed remains higher in the other portion. Then, in the posterior pass, beading occurs on almost the entire surface of the printing medium and the united ink droplets of the beading easily move according to the distribution of the permeation speed that occurred in the prior pass. As a result, the ink droplets that are united in the posterior pass may move a relatively large distance and may be fixed in a position that is different from the original position (pixel). That is, image unevenness generated in the prior pass causes the image unevenness of the posterior pass due to the distribution of the permeation speed. With such mechanism, a decrease of the number of passes to make printing speed higher includes a problem that density unevenness or color unevenness due to the beading becomes remarkable.
A method of adopting a dot arrangement having a high frequency characteristic can be considered as one of the methods of reducing the unevenness due to beading. This dot arrangement having the high frequency characteristic has little low-frequency components, and the distribution of beading that can be generated in an image printed in the prior pass has a short period (high frequency). As a result, the distance that the united ink moves in the posterior pass can be small, and the ink can be restrained from being fixed in a position that is different from the original position. However, because the dot arrangement having a high frequency characteristic has high periodicity, it can be easily influenced by the dot arrangement of other ink or external disturbances such as air currents and mechanical vibration, and other problem such as moiré patterns and stripes on an image may be remarkably brought about. In other words, an image of the dot arrangement having the high frequency characteristic is an image having low robustness.